I. Immunoglobulin structure and diversity. 1) Complete variable region amino acid sequences have been determined for 12 antibodies with specificity for Beta (1,6) galactan. Analysis of these sequences has permitted an assessment of diversity among IgM proteins and a definition of the structural basis of a number of idiotypes expressed on these molecules. 2) Sequence analysis of antibodies to phosphocholine has presented data suggesting gene conversion in immunoglobulin variable regions. The rearranged gene postulated to have undergone conversion has been cloned and sequenced and the nucleic acid data is consistent with the originally postulated conversion event. II. Cell membrane proteins: A number of genomic clones encoding porcine major histocompatability antigens have been isolated and are presently undergoing characterization. The nucleotide sequence of an expressed gene is nearing completion and appropriate comparisons are being made with MHC antigens from other species. These genes are also being tested for expression by DNA transfer experiments using mouse L cells.